1. Field of the Invention
The present invention relates to an output buffer circuit to be embodied in a semiconductor device and, more particularly, to an output buffer circuit having low noise characteristics, in which a ground bounce noise can be minimally generated.
2. Description of the Prior Art
In general, since a ground bounce noise makes electric potential of a ground terminal in a semiconductor device to be instable, it lowers the performance of overall circuit elements provided in the device.
Prior art output buffer adapted to describe such a ground bounce noise is shown in FIGS. 1 and 2. FIG. 1 is a schematic circuit diagram of a prior art output buffer.
As shown in FIG. 1, the prior art output buffer is constituted by two transistors each having a gate for receiving a signal S/A from a sense amplifier (not shown), one of which is a pull-up PMOS (P-channel Metal Oxide Silicon) transistor, and the other of which is a pull-down NMOS (N-channel Metal Oxide Silicon) transistor.
Equivalent circuit of the prior art output buffer is shown in FIG. 2. As shown in this drawing, a capacitance C is formed at the output terminal of the buffer, and inductances L1 and L2 are formed at the pull-up and pull-down transistors, respectively. In the construction of the buffer, a constant voltage V.sub.out is charged in the capacitance C during pull-up operation thereof, and a current flowing through the NMOS transistor is in proportion to the charged voltage V.sub.out during pull-down operation thereof. For example, relation between a driving voltage V.sub.DD and a magnitude of the current I flowing during the pull-down operation can be expressed by following formula: ##EQU1##
As can be seen from the formula, the current I is in proportion to the voltage V.sub.out. Then, a voltage V.sub.GB between both ends of the inductance L2 formed at the NMOS transistor, i.e. a ground bounce voltage, causes a noise to make the level of total grounds of the semiconductor device instable, as given by following formula. ##EQU2##
In the prior art output buffer having a PMOS transistor serving as a pull-up transistor, however, when a driving voltage is more than a normal voltage, the amount of a current flowing through a pull-down PMOS transistor is increased during pull-down operation thereof, as can seen from the above formulas. Thus, a ground bounce voltage is also increased enough to cause a significantly heighten noise.